﻿using System;
using System.Collections.Generic;
using System.Text;

namespace TemplateTool.SnowflakeId
{
    public class Snowflake
    {
        private static long machineId;//机器ID
        private static long datacenterId = 0L;//数据ID
        private static long sequence = 0L;//计数从零开始
        private static long twepoch = 687888001020L; //唯一时间随机量

        private static long machineIdBits = 5L; //机器码字节数
        private static long datacenterIdBits = 5L;//数据字节数
        public static long maxMachineId = -1L ^ -1L << (int)machineIdBits; //最大机器ID
        private static long maxDatacenterId = -1L ^ -1L << (int)datacenterIdBits;//最大数据ID

        private static long sequenceBits = 12L; //计数器字节数，12个字节用来保存计数码        
        private static long machineIdShift = sequenceBits; //机器码数据左移位数，就是后面计数器占用的位数
        private static long datacenterIdShift = sequenceBits + machineIdBits;
        private static long timestampLeftShift = sequenceBits + machineIdBits + datacenterIdBits; //时间戳左移动位数就是机器码+计数器总字节数+数据字节数
        public static long sequenceMask = -1L ^ -1L << (int)sequenceBits; //一微秒内可以产生计数，如果达到该值则等到下一微妙在进行生成
        private static long lastTimestamp = -1L;//最后时间戳

        private static object syncRoot = new object();//加锁对象
        static Snowflake snowflake;

        public static Snowflake Instance()
        {
            if (snowflake == null)
                snowflake = new Snowflake();
            return snowflake;
        }

        public Snowflake()
        {
            Snowflakes(0L, -1);
        }

        public Snowflake(long machineId)
        {
            Snowflakes(machineId, -1);
        }

        public Snowflake(long machineId, long datacenterId)
        {
            Snowflakes(machineId, datacenterId);
        }

        private void Snowflakes(long machineId, long datacenterId)
        {
            if (machineId >= 0)
            {
                if (machineId > maxMachineId)
                {
                    throw new Exception("机器码ID非法");
                }
                Snowflake.machineId = machineId;
            }
            if (datacenterId >= 0)
            {
                if (datacenterId > maxDatacenterId)
                {
                    throw new Exception("数据中心ID非法");
                }
                Snowflake.datacenterId = datacenterId;
            }
        }

        /// <summary>
        /// 生成当前时间戳
        /// </summary>
        /// <returns>毫秒</returns>
        private static long GetTimestamp()
        {
            return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
        }

        /// <summary>
        /// 获取下一微秒时间戳
        /// </summary>
        /// <param name="lastTimestamp"></param>
        /// <returns></returns>
        private static long GetNextTimestamp(long lastTimestamp)
        {
            long timestamp = GetTimestamp();
            while (timestamp <= lastTimestamp)
            {
                timestamp = GetTimestamp();
            }
            return timestamp;
        }

        /// <summary>
        /// 获取长整型的ID
        /// </summary>
        /// <returns></returns>
        public static long Id
        {
            get
            {
                lock (syncRoot)
                {
                    long timestamp = GetTimestamp();
                    if (lastTimestamp == timestamp)
                    { //同一微妙中生成ID
                        sequence = sequence + 1 & sequenceMask; //用&运算计算该微秒内产生的计数是否已经到达上限
                        if (sequence == 0)
                        {
                            //一微妙内产生的ID计数已达上限，等待下一微妙
                            timestamp = GetNextTimestamp(lastTimestamp);
                        }
                    }
                    else
                    {
                        //不同微秒生成ID
                        sequence = 0L;
                    }
                    if (timestamp < lastTimestamp)
                    {
                        throw new Exception("时间戳比上一次生成ID时时间戳还小，故异常");
                    }
                    lastTimestamp = timestamp; //把当前时间戳保存为最后生成ID的时间戳
                    long Id = timestamp - twepoch << (int)timestampLeftShift
                        | datacenterId << (int)datacenterIdShift
                        | machineId << (int)machineIdShift
                        | sequence;
                    return Id;
                }
            }
        }
    }


}
